Twitter

The first thing +1 will likely do is help Google improve it search results. But after that, Google could take a user's incoming stream of +1s and send them to all that user's friends – kind of like the way Facebook sends a user's "likes" to that user's friends' "News Feeds."

Google wants to create that kind of stream because increasingly, people are finding things on the Internet in their Facebook and Twitter streams BEFORE they ever get a chance to search for them on Google.

To make +1 work, though, Google needs to seed it with a "social graph." Facebook has one, but it's not for sale and Google couldn't buy it anyway.

Twitter has a much smaller one, and it might be Google's best bet.

It might be a better fit too: Twitter is less about sharing social information and more about sharing content – like Google.

Twitter has long been unwiling to sell to Google, but things are different now. For one, Twitter CEO Dick Costolo is a professional CEO, not a founder. He could be bought. For another, we keep hearing that company is a bit of a mess right now – from the product organization all the way up to the board. If Page comes calling with $7 billion, Twitter is his.

Foursquare

Google would buy Foursquare because it has a real and growing "social graph" – though it's still very small – and because it would be simple to have Google's massive sales force start selling "deals near you" coupons for Foursquare users.

Foursquare doesn't help Google with the whole people-are-finding-Web-content-before-searching problem, though.

By the way, everyone thinks Dennis Crowley won't sell to Google because he sold his first company, Dodgeball, to Google and Google killed it a couple years later.

That's wrong.

Crowley HAD to sell Dodgeball back then because he couldn't find VC. These days, Crowleys knows that Google is one of the few companies out there that would write a $1 billion check. So he's not ignoring the Google option – especially since Page seems willing to let entreprenuers run their own businesses under the Google umbrella.